7-[(Chlorocarbonyl)methoxy]-4-methylcoumarin 1 has been proposed for fluorescent labeling of primary and secondary hydroxyl groups. We have developed synthetic procedures which allow the gram-scale production of 1 in high purity and we have thoroughly characterized this acid chloride by mass spectrometry and multinuclear and multi-dimensional nuclear magnetic resonance spectroscopy. Reagent 1 reacts smoothly with a variety of primary and secondary hydroxyl compounds to give the corresponding esters. Although derivatives of 1 readily elute from reversed-phase HPLC columns, analyses of these compounds by capillary GC/MS were unsuccessful for all but the lowest molecular weight materials due to limited volatility. Although there are reports of the use of CI and FAB MS in the examination of compounds of similar structure, we find that EI (70 eV) MS provides the most extensive indication of chemical structure. All derivatives give a distinctive molecular ion (relative abundance 100%-12%), and ions at 189 which are indicative of the 7- carbonylmethoxy-4-methylcoumarin moiety. Loss of molecular carbon monoxide (CO) occurs unpredictably. Finally, for those compounds possessing a suitable structure in the alcohol portion of the ester, the McLafferty Rearrangement occurs, leading to a characteristic ion at m/z 234.